Confluence user license was temporarily switched off for a user account while their Jira license remained activated. However, calls to the Jira REST API to create Jira issues started returning the following error:
"Failed to create JIRA ticket: Field 'assignee' cannot be set. It is not on the appropriate screen, or unknown."
Confluence user license was switched back on for this user account but the above error message continues to be returned when attempting to call the Jira REST API to create Jira issues.